This method of editing uses two reels of film with common cue marks to make a master print out of them. One reel contains images up to the first effect, and then blank leader to leave space for the next set of scenes, while the second reel contains blank leader which is the same time as the scenes in reel A up to the first effect. This continues, alternating where the images and leader are, such that the film can easily be edited together into the master print. The system is also useful in eliminating visible splices in the editing of 16mm film.

The Film Encyclopedia